Inductively coupled plasma-atomic emission spectrometry (ICP-AES) is one of the most commonly used techniques for simultaneous trace multi-element analysis. However, the method is not without its shortcomings. One of the important remaining challenges is the understanding and elimination of matrix effects. Matrix effects occur when the detailed chemical composition of the sample and standard solutions are not identical; the signal strength of a particular emission line from the analyte at an identical analyte concentration changes with the concentration of other constituents (the “matrix”) in the sample. As a result, analyses carried out through simple external calibration can lead to bias and analytical inaccuracy.
The behavior and mechanisms of matrix effects have been the subject of many studies. Matrix effects found in ICP-AES, according to their origins, can be divided into the three following categories: (a) spectral interferences, (b) sample introduction related, and (c) plasma related. The presence of any such interferences will typically cause error in the analysis.
Spectral interferences arise from the imperfect isolation of desired spectral lines from the composite radiation that passes the instrumental spectral window tuned to those lines. Spectral interferences, especially line overlaps, can be a significant problem in ICP-AES. Although efforts have been made to collect spectral interference data in different matrices or to build in databases in commercial ICP-AES instruments to help users select emission lines free from spectral interference in different matrices, the selection of an appropriate analytical line can still be difficult, especially for a sample with a complex or unknown matrix. Improper line selection results in a loss of detection power and analytical errors. Analytical errors caused by direct-line overlap spectral interference are difficult to recognize because the overlapping emission lines appear as one and become indistinguishable.
A currently available method to detect the presence of direct spectral-line overlap is through checking with a spectral-line database if the concentration of the matrix elements in the “unknown” sample is approximately known. These databases list all known emission lines from a particular matrix (although they may be incomplete), so an analyst can avoid employing an emission line with potential spectral-interfering wavelength positions in the analysis.
Moreover, if more than one emission line is employed for the analyte element, the emission line that suffers from spectral interference will give a significantly different (i.e., beyond experimental measurement uncertainty) analytical result than the others. However, if only two emission lines are used (in this case, results from the two emission lines are different but one cannot easily tell which one is correct), or if a few chosen emission lines suffer from spectral interferences (in this case, analytical results from different emission lines scatter and it can be difficult to determine which are the “outliers”), it may be difficult to know which emission line is free from spectral interference without referring to the wavelength database together with knowledge of the sample matrix.
For solution samples (the most common sample-introduction method for the ICP), sample-introduction-related matrix interferences are typically related to the aerosol formation or transport processes. Aerosol formation is the process of disintegration of a bulk liquid into small droplets for transport into the ICP by the carrier gas. Aerosol generation and the resultant aerosol droplet size distribution are related to the physical properties of the solution; in particular density, viscosity, and surface tension. In the presence of a matrix, these physical properties might change and result in a change in aerosol droplet size distribution. As a result, the rate of analyte injection into the plasma will be changed, and hence the sensitivity of the measurements.
For example, it has been found that acids such as sulfuric acid and phosphoric acid give rise to coarser primary aerosols because they increase the viscosity of the solutions. Although there are many possible routes by which the nebulization and transport processes can be affected by a matrix, detection of the presence of sample-introduction-related matrix effects and their subsequent correction are rather straightforward. A change of solution uptake rate, aerosol generation and transport efficiency will affect the accuracy of all elements (analytes) to the same extent, regardless of the physical properties of the elements. Thus, such interferences may be easily corrected by an arbitrary single internal standard spiked into the samples, provided that the added internal standard is chemically compatible with the sample and that the sample originally does not contain (or alternatively, contains a consistent amount of) that spiked element.
Plasma-related matrix interferences have been the focus of many studies. For plasma-related matrix interferences, the degree of interference usually depends on the characteristics of the analyte and its emission line. As a result, it is not possible to use a single internal standard for universal correction in a multi-element analysis. Consequently, a wide variety of techniques and parameters have been explored and developed to probe plasma-related matrix interferences. However, most such techniques (e.g., Rayleigh and Thomson scattering, ratios of (laser induced) atomic fluorescence to atomic emission, and computed tomography) are too complicated and instrument-demanding to be applicable to a commercial ICP-AES spectrometer intended for routine analysis.
For indicators that can be easily adaptable to a commercial instrument, the indicator should likely be based solely on the measurement of emission intensities or spectral line widths. The only effective emission-based matrix-effect indicator currently used is the ionic-to-atomic line-intensity ratio. The Mg II/Mg I ratio is commonly used to gauge the robustness of the plasma (its susceptibility to matrix interference) under different operating conditions, and even among different ICP instruments. Although very successful and applicable in many situations, the technique suffers two potential limitations. This technique requires either that the test element (e.g., Mg) can be artificially added (if not originally present in the sample), and also that it be within the linear dynamic range of the instrument (if the element is already present in the sample). Taking Mg as an example, Mg-rich samples are abundant (e.g., environmental or botanical samples) but it has been reported that the Mg II/Mg I ratio is not adequate to reflect plasma robustness for such samples because of the limited linear dynamic range of the method.
Moreover, it requires that the chosen pair of emission lines suffers no spectral interference from other constituents in the sample. Spectral interference from other constituents in the sample can be a potential problem. There is only one strongly emitting Mg I line and if any other spectral line overlaps with it, the calculated Mg II/Mg I ratio is erroneous. Although an ionic and neutral-atomic line pair from a more line-rich element might be substituted for Mg, most elements exhibit only weak neutral-atomic emission lines in the ICP. Moreover, emission from each spectral line is generally weaker for a line-rich element, so a higher concentration of the test element is required to avoid spectral interference from other constituents of the sample.
Once the presence of a plasma-related matrix interference is noted, conventional methods exist to cope with such issues. Four conventional techniques have been used: matrix separation; matrix matching between the sample and the standards; by the use of internal standards; and by standard additions. While these techniques can solve the problem of matrix interference when correctly applied, there are shortcomings.
For example, matrix separations are typically laborious and can lead to analyte loss. Also, the identity of the interfering matrix needs to be known before an efficient separation method can be applied. Matrix-matching requires knowledge of the approximate concentrations of the major sample constituents and therefore, may require an additional pre-analysis. Internal standardization requires a prior-analysis study to find the correct internal standard(s) that change similarly with the analyte and, in general, more than one internal standard is usually required in a multi-element analysis. Standard addition requires spiking (“addition”) of the analyte into the sample and several additions are usually required for a single sample, which considerably lengthens the total analysis time. Moreover, it has been shown through statistical analysis that the concentration ratios of the spiked sample and the original sample have a critical impact on the precision of the x-intercept of the standard-addition curve through extrapolation (i.e., the final analysis result).
It has been shown that the concentration of the analyte in the spiked sample should preferably be at least two times the original concentration and that precision degrades in an exponential fashion when the spiked concentrations are lower than the original concentration. Also, the concentrations of the spiked analyte should not be too high as the principle of standard addition assumes that the matrix is invariant and that there are identical matrix effects in both the original and the standard-spiked samples. This assumption can be met only if the added standard is not too large compared to the original concentration of the analyte (i.e., roughly within one order of magnitude). This problem is particularly acute considering that ICP-AES is a simultaneous multi-element analysis technique, which means that multi-element spikes must be added concurrently.
Therefore, the margin is comparatively narrow for the concentration of the spike to be added and the absolute amount is clearly sample-dependent. In other words, to yield results with reliable precision and accuracy, the concentration of the analyte in the sample should be known semi-quantitatively. To summarize, all four of these conventional techniques require either a separate study or some prior knowledge about the unknown sample, which requires an additional pre-analysis to obtain such information. Currently, there is no on-the-fly method available to compensate for plasma-related matrix interference for a completely unknown sample. In addition, all of the above-mentioned techniques require the addition of foreign reagents to the sample and, therefore, are more prone to human error and chemical impurities.
Another major source of analytical errors in ICP-AES is instrumental drift. The result of such drift is the need for frequent recalibration, which is time-consuming. It is common in routine analysis to run regularly a so-called quality-control (QC) sample of predefined concentration in order to monitor system drift, and to serve as a decision tool as to whether recalibration is required. If drift is found between two calibrations, there is no general rule to correct the intervening data to compensate for the drift; although it is often assumed that drift is linear with time, that assumption is not always valid. Although running QC samples is effective for monitoring system stability, it can be difficult to determine how often a QC sample must be run. Preferably, instrumental drift correction is performed in an on-line fashion together with the analysis of an unknown sample. With this approach, immediate corrective action can be taken once a drift is noted.